This invention relates in general to imaging airway geometry and more particularly concerns noninvasively obtaining a signal representative of the cross-sectional area of an airway (e.g., oral, nasal, or pulmonary) of a subject (e.g., a person or an animal) using multiple spaced electroacoustical transducers.
A one-dimensional image of the cross-sectional area of an airway as a function of axial position along the airway may, be determined from acoustic reflections measured by a single electroacoustic transducer placed in a position remote from the airway opening. This image is referred to as an area-distance function and is represented by A(x) where x is the axial position along the airway.
Knowledge of the area-distance function, A(x), is useful in the diagnosis of pathologies associated with oral airways, larynx, pulmonary airways, and nasal airways, for example. These pathologies include but are not limited to obstructive sleep apnea, asthma, obstructive pulmonary disease, tracheal stenosis, and nasal septum deviation. Accurate information about the area-distance function is also useful in the study of airway growth and its disruption and sequelae of bronchopulmonary dysplasia in children, for example.
One approach toward using a single electroacoustic transducer is described in U.S. Pat. No. 4,326,416 granted Apr. 27, 1982, to Jeffrey J. Fredberg entitled ACOUSTIC PULSE RESPONSE MEASURING. A two-transducer approach is described in a paper of M. R. Schroeder entitled xe2x80x9cDetermination of the Geometry of the Human Vocal Tract by Acoustic Measurementsxe2x80x9d in J. Acoust. Soc. Am. 41(4), 1002-10 (1967).
According to the invention there is apparatus for providing an output signal characteristic of the geometry of a confined volume including a conduit for exchanging acoustical energy with the confined volume. The conduit has an open first end in communication with an opening in the confined volume. A transducer, such as a loudspeaker, is coupled to the conduit for launching acoustical energy into the conduit producing an incident wave towards the opening in the confined volume and a reflected wave to form a wave field in the conduit representative of the geometry of the confined volume.
In accordance with one feature of the invention at least one pressure wave sensing transducer, such as a microphone, is mounted along the length of a conduit. The pressure wave sensing transducer is mounted in spaced relationship to provide a transduced signal representative of the wave field. The conduit has a pair of open ends. One of the ends is open to the atmosphere, an environment at substantially atmospheric pressure, another instrument such as a flowmeter, volume meter (i.e., spirometer) or a ventilator. The other end is adapted for coupling to the confined volume. A transducer, such as a loudspeaker, is coupled to a sidewall of the conduit for launching acoustical energy into the conduit producing an incident wave towards the opening in the confined volume and a reflected wave to form a wave field in the conduit representative of the geometry of the confined volume. A processor processes the transduced signal to provide an output signal characteristic of the geometry of the confined volume, such as the cross-sectional area of the confined volume as a function of the distance from the opening in the confined volume.
In accordance with another feature of the invention at least first and second pressure wave sensing transducers, such as microphones, are mounted along the length of the conduit. The conduit may be either lossless (i.e., free of sound absorbing or acoustic energy absorbing material terminations), or may include sound absorbing material terminations. In either case, reflections from an end of the conduit are processed to provide a signal representative of the impulse response of the airway. The at least first and second pressure wave sensing transducers are mounted in spaced relationship to provide first and second transduced signals representative of the wave field. The conduit has a pair of open ends. One of the ends is open to the atmosphere, an environment at substantially atmospheric pressure, another instrument such as a flowmeter, volume meter (i.e., spirometer) or a ventilator. The other end is adapted for coupling to the confined volume. A transducer, such as a loudspeaker, is coupled to a sidewall of the conduit for launching acoustical energy into the conduit producing an incident wave towards the opening in the confined volume and a reflected wave to form a wave field in the conduit representative of the geometry of the confined volume. A processor processes the first and second transduced signals to provide an output signal characteristic of the geometry of the confined volume, such as the cross-sectional area of the confined volume as a function of the distance from the opening in the confined volume.
The process according to the invention includes connecting an open first end of the conduit to an opening in the confined volume, delivering acoustical energy into the conduit to provide incident and reflected waves that form the wave field in the conduit, transducing acoustic energy at spaced locations along said conduit to provide first and second transduced signals representative of said wave field at spaced locations in the conduit, and processing said first and second transduced signals to provide an output signal representative of the geometry of said confined volume. The second open end allows a portion of reflected wave energy to pass to the atmosphere, an environment at substantially atmospheric pressure, another instrument such as a flowmeter, volume meter (i.e., spirometer) or a ventilator.
The invention provides an output signal representative of airway geometry without calibration in apparatus that is relatively small and portable. The invention may be used for diagnostic and screening purposes in a confined area such as a laboratory, a doctor""s office, a place of work, and at bedside. Additionally, the invention requires little or no cooperation by the subject being tested, facilitating its use in pediatric applications. Further, because the second end is opened to the atmosphere, an environment at substantially atmospheric pressure, another instrument such as a flowmeter, volume meter (i.e., spirometer) or a ventilator, the arrangement is more comfortable for the patient.
In accordance with still another feature of the invention, low frequency components in either the acoustic energy delivered to the conduit, or in the processing of the transduced signals, are removed by high-pass filtering. The high-pass filtering may be used with either a single pressure-wave-sensing transducer arrangement or the arrangement using at least first and second pressure-wave-sensing transducers. The high-pass filtering reduces errors associated with acoustic waves reflected by nonrigid airway walls without the necessity for a high acoustic-wave-speed gas, such as Hexe2x80x94O2. Thus, with the high-pass filtering, use of a more convenient gas, such as air, albeit of lower acoustic wave speed, may be used and still provide reduced, nonridged airway wall associated errors.